I'm very happy with ESXi 6.0. It's the only version of ESXi so far where all of the features I use actually work right out of the box.
Thus, I was planning to skip over 6.5 entirely and wait for the next version, but as 6.7 is incompatible with Xeon 55xx CPUs, it seems 6.5 is the end of the line for my server. If I'm not planning on using any of the new features in 6.5, is there any reason to upgrade?
Since everything is working fine and You are not going to utilizing the new features in 6.5.
You could continue to use ESXI version 6.0 (latest build).
You are correct 6.7 is incompatible with Xeon 55xx CPUs series.